1. Field of the Invention
The present invention relates in general to systems for detecting undesirable conditions of an internal combustion engine, and more particularly to systems for detecting knocking of the engine.
2. Description of the Prior Art
As is known, engine knocking caused by rapid preflame reactions within the end gases lowers the output and the thermal efficiency of the engine.
In order to reduce the knocking, various measures are commonly employed, which are, for example, usage of a fuel having a higher octane rating, lowering of combustion pressure and/or combustion temperature, reducing of the time during which the end gases are under high pressures and temperatures. However, these measures have failed to exhibit satisfied results.
Thus, recently, there have been proposed systems in which a knocking sensor is employed for detecting the undesired engine knocking, and in which when an engine knocking is detected by the knocking sensor, a spark advancer is actuated to delay the ignition timing thereby suppressing or at least minimizing the engine knocking.
One of such systems is shown in Japanese Patent First Provisional Publication No. 64-54227. In the system of this publication, an engine vibration representative signal issued from a knocking sensor is passed through a band-pass filter means. The filtered signal from this filter means is compared with a reference signal to determine whether the engine is under knocking or not. The band-pass filter means is so controlled as to narrow the pass-band thereof for minimizing noise possessed by the filtered signal. In fact, if the pass-band of the filter is not narrowed enough, the S/N ratio of the filtered signal increases and thus the detection of a small knocking of the engine becomes difficult.
In the disclosed system, the filter means consists of a knocking controlling band-pass filter whose pass-band is variable and two additional band-pass filters which control the pass-band of the knocking controlling band-pass filter. The pass-band change is carried out based on the peaked outputs of the three band-pass filters which are obtained after the knocking detection by the knocking controlling band-pass filter, and thus, the frequency pass-bands of these three filters are overlapped.
That is, for the pass-band change, with reference to the peaked levels of the engine vibration representative signals from the filters, the center frequencies of the three band-pass filters are weighted to obtain a mean frequency which is treated as a practical knocking frequency. In fact, the mean frequency is used as the center frequency of the knocking controlling filter.
Although, as is described hereinabove, the disclosed system can reduce the noise of the filtered signal by narrowing the frequency pass-band, the same has the following drawback due to its inherent construction.
That is, in the disclosed system, the knocking occurrence within the narrowed pass-band is absolutely necessary. However, as is known, a knocking representative frequency is not always contained within the narrowed pass-band, and thus satisfied knocking detection is not expected from such conventional system.